Disk drives are capable of storing large amounts of digital data in a relatively small area. Disk drives store information on one or more recording medias, which conventionally take the form of circular storage disks having a plurality of concentric circular recording tracks. A typical disk drive has one or more disks for storing information. This information is written to and read from the disks using read/write heads mounted on actuator arms that are moved from track to track across the surfaces of the disks by an actuator mechanism.
Generally, the disks are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the disks under the read/write heads. The spindle motor generally includes a shaft mounted on a base plate and a sleeve, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the sleeve interact with a stator winding on the base plate to rotate the sleeve relative to the shaft. In order to facilitate rotation, one or more bearings are usually disposed between the sleeve and the shaft.
From the foregoing discussion, it can be seen that the bearing assembly which enables the storage disk to rotate is of critical importance. One bearing design is a fluid dynamic bearing. In a fluid dynamic bearing, a lubricating fluid such as oil, among other fluids, provides a bearing surface between fixed and rotating members of the motor. Fluid dynamic bearings spread the bearing surface over a large surface area, as opposed to a ball bearing assembly, which comprises a series of point interfaces. This bearing surface distribution is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing, which helps to reduce non-repeatable run-out. Thus, fluid dynamic bearings are an advantageous bearing system.
The internal lubricating fluid volume of fluid dynamic bearings utilized in FDB motors must be substantially free of trapped air during motor operation to maximize drive performance. External motor reservoirs typically do not have the capacity to hold the total required lubricating fluid volume. Thus, air trapped must be removed before filling the internal lubricating fluid volume of fluid dynamic bearings.
In order to remove the air trapped within the bearing, a vacuum filling process has been developed where the FDB motor is partially filled before a vacuum is applied to extract air from the internal areas of the bearing. The remaining lubricating fluid is added and the vacuum is released, thereby drawing the fluid into the motor. However, during the application of the vacuum, air may be violently expelled from the bearing, causing lubricating fluid within the bearing to exit the motor. Additionally, the escaping air may create an air channel void of lubricating fluid necessary to fill internal area when vacuum is released, and thus the filling process fails as trapped air is not totally removed.
Moreover, current fluid dynamic bearing designs generally include a space for lubricating fluid expansion and therefore a portion of the total available lubricating fluid reservoir volume must remain unfilled. However, entrained air and lubricating fluid will fill almost 100 percent of the bearing reservoir volume. At this point, the bearing does not have any additional space to accommodate expansion in fluid volume due to temperature change.
Therefore, a need exists for a method and apparatus for efficiently filling fluid dynamic bearings.